1. Field of the Invention
The present invention relates to a gas sensor controller for controlling a gas sensor element which detects the concentration of a specific gas, a gas sensor system which includes the gas sensor controller and a gas sensor, and a deterioration determination method for determining the deterioration state of the gas sensor element.
2. Description of the Related Art
Conventionally, a gas sensor has been known which includes a gas sensor element for detecting the concentration of a specific gas contained in, for example, the exhaust gas of an internal combustion engine, and a gas sensor controller for controlling the gas sensor element. For example, Patent Document 1 discloses an NOx sensor which includes a gas sensor element (NOx sensor element) for detecting nitrogen oxides (NOx) as a specific gas, and a gas sensor controller which controls the gas sensor element and calculates the concentration of nitrogen oxides contained in an object gas.
Notably, the NOx sensor element used in the NOx sensor has first and second measurement chambers which respectively have a first pump cell and a second pump cell each formed of a solid electrolyte body.
The object gas is introduced into the first measurement chamber, and the first pump cell controls the first chamber gas within the first measurement chamber to a predetermined oxygen concentration. One of the paired electrodes of the second pump cell is disposed in the second measurement chamber. When a predetermined voltage is applied to the second pump cell, oxygen molecules and oxygen-containing gas molecules including oxygen atoms in their respective structures (e.g., NOx) introduced into the second measurement chamber from the first measurement chamber are disassociated. As a result, a concentration current corresponding to the concentration of the oxygen molecules and oxygen-containing gas molecules contained in the second chamber gas flows between the electrodes of the second pump cell. Therefore, the gas sensor controller can detect the concentration of the oxygen-containing gas from the magnitude of the concentration current flowing between the electrodes of the second pump cell.
[Patent Document 1] U.S. Patent Application Laid-Open No. 2009/0164091
3. Problem to be Solved by the Invention
When the gas sensor element used in such a gas sensor deteriorates due to use or other causes, it has been found that the response of the sensor becomes slow. For example, an NOx sensor must have a good response characteristic in order to cope with recent strict NOx regulations. However, if the response of the sensor becomes slow due to deterioration of an NOx sensor element used in the NOx sensor, the NOx sensor may fail to satisfy the required response characteristic. Therefore, there has been a demand for a gas sensor controller which can determine the deterioration state of such a gas sensor element whose response has become slow.